This relates generally to image sensors, and more specifically to back-side-illuminated (BSI) image sensors with conductive bias grids to enhance charge collection and improve photodiode isolation.
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. Conventional image sensors are fabricated on a semiconductor substrate using complementary metal-oxide-semiconductor (CMOS) technology or charge-coupled device (CCD) technology. The image sensors may include an array of image sensor pixels each of which includes a photodiode and other operational circuitry such as transistors formed in the substrate.
Image sensors often include a photodiode having a pinning-voltage which is a design parameter set by the doping levels of the photodiode. During normal operation, a photodiode node is first reset to the pinning-voltage using transistor circuitry. Photons are then allowed to enter the photodiode region for a given amount of time. The photons are converted to charge carriers inside the photodiode, and these charge carriers reduce the reset pinning-voltage. In this process, the maximum total charge stored, QMAX, is commonly referred to as the saturation full well (SFW) and depends on the well capacity of the photodiode. The actual charge stored, Q, is less than or equal to QMAX based on the intensity and integration time of photons. When it is time to read out the stored signal, the stored charge Q at the photodiode node is transferred to a floating diffusion node through additional transistor circuitry. If care is not taken to maximize the amount of charge Q than can be transferred from the photodiode to the floating diffusion node, charge spill back can degrade image quality. Maximum charge stored, QMAX, determines the highest signal level detected in the photodiode array. High QMAX improves dynamic range of an image sensor, lowers the noise floor, and can be used to improve saturation artifacts.
A deep photodiode (DPD) process can be used to increase the quantum efficiency and QMAX characteristics of a pixel, which have an increasing impact on pixel performance as pixel area decreases. In conventional image sensors, the electric field strength of ground electrodes in a pixel is weakened as the substrate depth of the pixel increases. The DPD process requires a thicker substrate than traditional pixel depth processes and also requires the addition of an additional resistive path, which further decreases the electric field magnitude of the ground contacts as substrate depth increases. As a consequence of the decreased electric field strength, the p-n junction close to the back-side interface of the pixel is not in strong reverse bias, which leads to low charge collection efficiency and weak electrical isolation. The lowered charge collection efficiency occurs due to an increased probability for charge carriers to recombine before reaching the ground contacts. Electrical crosstalk and blooming may occur as a result of weak electrical isolation.
It would therefore be desirable to be able to provide BSI image sensors with enhanced charge collection and improved photodiode isolation.